The present invention relates to a gradient magnetic field generator in an MRI (Magnetic Resonance Imaging) system using an NMR (Nuclear Magnetic Resonance) phenomenon, and more specifically, to the improvement in reducing noise caused by pulsed electromagnetic forces exerted on gradient coils.
It is well known that a medical MRI system includes a bed device for putting a patient thereon, a magnet device for forming a static magnetic field, a transmitter-receiver for transmitting a high frequency wave for generating magnetic resonance to the patient and for receiving an MR signal from the patient to detect and amplify the MR signal, a gradient magnetic field generator for discriminating the position of the MR signal and a processor-controller for controlling the entire system and reconstructing the image.
The gradient magnetic field generator has a gradient coil unit and a gradient magnetic field power supply. The gradient coil unit comprises three groups of coils commonly called as x-coils, y-coils and z-coils for forming gradient magnetic fields linearly changed in their intensities in respective coordinate directions, with supplied pulse currents from the gradient magnetic field power supply.
One type of the gradient coil unit is formed by mechanically fixing a plurality of x-coils, y-coils and z-coils to be paired with respect to the Z-direction (i.e. the longitudinal direction of the unit), respectively, on the outer peripheral side of a bobbin made of a fiber reinforced composite material (hereinafter referred to as FRP) impregnated with non-magnetic resin. Also, another type of the gradient coil unit is formed by winding x-coils, y-coils and z-coils on the outer peripheral surface side of an internal cylinder made of FRP, surrounding the outside thereof by another cylinder made of FRP, and impregnating the gap between the two cylinders with resin into an integrated structure, thereby winding the x-coils, y-coils and z-coils in the state of embedding them within a bobbin.
The gradient magnetic field power supply is to be a pulse current source for supplying a pulse current with a pulse of a several millseconds in its first and last transitions to x-coils, y-coils and z-coils through operation of a processor-controller.
Such MRI system as mentioned above has been disadvantageous in that, as a pulse current is applied from the gradient magnetic field power supply to the x-coils, y-coils and z-coils in a static magnetic field, the coils (such as flat lead wires) receive pulsed eletromagnetic forces in various directions, which causes the deflection of the bobbin resulting in the generated noise.
In order to reduce the noise, there has been proposed such a technique that a sound absorption material is wound on the outer peripheral side of the gradient coil unit, or is inserted between a superconductive magnet as a magnet device and a frame cover. However, in the coil attachment structure of the above mentioned gradient coil unit, the coil attachment is restricted so as not to increase the rigidity, and also the gradient coil unit itself is low in its rigiditiy. Therefore, the noise generating energy is left as being high, and also if being absorbed by a sound absorption material, sound components not to be absorbed and to be leaked are left. They are transferred to patients in a diagnostic dome as non-continuation sound of about 70 to 80 phons, thus still bringing inhospitable feeling on the patients.
Another technique for reducing the noise is that prefabricated saddle-shaped x-coils and y-coils and ring-shaped z-coils are rigidly combined by using support elements to form a coil basket and the coil basket is supported via insulating elastic support elements on a hollow cylindrical body. However, forming the coil basket and attaching the coil basket to the hollow cylindrical body via the elastic support elements leads to increased size of the gradient coil unit in the radial direction and increasd assembly time.